For rapid printers and similar equipment, which are used, inter alia, as output means in computer systems, the printing medium is usually a paper web with perforations at its edges for advancing the web. The movement of the latter in relation to the print position of the printer is obtained by advancing means provided with teeth cooperating with the perforations in the paper such as to advance the web forwards or backwards relative the print position. Advancing means of this type, so-called tractor feeders, are commercially available and are known, inter alia, from the U.S. Pat. No. 3,825,162.
Tractor feed is mainly of two types, single or push-pull traction. With the former the paper web is moved with the aid of the tractor feeder via conducting means past a cylindrical or flat platen, against which printing is performed. In push-pull traction the feed to the print position takes place in the same way as for single traction, but down-stream of the print position the perforations in the paper web are brought into engagement with another section of the feeder where the teeth move in the opposite direction to those advancing the paper web towards the print position, thus obtaining an advance away from the print position as a supplement to the paper advance in a singlefeed feeder.
Single traction feed is mainly used for printing forms such as tickets, consignment notes and the like. Push-pull traction feed is customarily used in printing on a continuous paper web, for such as computerized lists. A problem in printing forms is that of being able to tear off the form as close to the print position as possible. In push-pull traction feed there are difficulties in replacing the paper web for renewed pulling on the down-stream side of the traction feeder, which pulls the web out of the printer.